Method and apparatus for impedance matching in an amplifier using lumped and distributed inductance

ABSTRACT

An impedance matching circuit ( 140 ) includes a capacitive element (C 1   , 220 ), having a capacitance C, coupled in parallel with an output node ( 215 ) of the matching circuit, and an inductor (L 1   , 225 ) coupled in series with a transmission line (T 1   , 230 ) between the input node and the output node. The transmission line has a length that, in combination with the inductor, provides impedance substantially equal to the input impedance of the transmission circuit ( 150 ) at a frequency of interest. In one embodiment, the inductor is connected to an output ( 195 ) of an amplifier ( 180 ), and the transmission line is connected to the inductor and to the output ( 215 ). The capacitive element is connected to the transmission line such that the length of the transmission line between the inductor and the capacitive element provides the desired inductance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to impedance matching toamplifiers, and more particularly to an impedance matching circuit andmethod for matching an output impedance of an amplifier circuit in awireless communications system to an input impedance of a transmissioncircuit.

2. State of the Art

Wireless communication systems typically include a chain of amplifiercircuit through which a received or modulated signal is passed inseries. The output of the amplifier stages is coupled to a load,typically via an impedance matching circuit.

Impedance matching circuits help match the output impedance of theamplifier stages to the impedance of the load. An ideal impedance matchprovides for the maximum transfer of power from the source to the load.In a wireless communication system, for example, it is typicallydesirable to maximize the power delivered from a final amplifier circuitor power amplifier to an antenna. The maximum power is transferred fromthe power amplifier to the antenna when, for a given frequency, theinput impedance of the antenna is equal to the conjugate of the outputimpedance of the power amplifier. When these conditions are satisfiedpower is delivered with 50% efficiency, that is as much power istransferred to the antenna as is dissipated in the internal impedance ofthe power amplifier.

Generally, the output impedance of a power amplifier will not be what isneeded for maximum power transfer. For example, a typical poweramplifier used in a handset of a wireless communication system may havean internal impedance of about 3 ohms, whereas the antenna used in thesame handset has an input impedance of about 50 ohms. Typically, amatching network comprising capacitors and inductors is inserted betweenthe power amplifier and the antenna to make the power amplifier outputimpedance appear to be the complex conjugate of the input impedance ofthe antenna.

But conventional matching circuits may require non-standard values for Land C to achieve a proper impedance match. Consequently, it generally isnot possible to design and build a matching network that preciselymatches the impedance of the power amplifier to that of the antenna Thisproblem is exacerbated by the variations in values of the inductors andcapacitors due to uncertainties or tolerances in their manufacturingprocesses. Inductors and capacitors can vary by 10% or more from theirspecified value. Accordingly, the larger the values of the inductors andcapacitors the larger the impact of the manufacturing tolerances on theimpedance matching. Thus, implementing a matching network to preciselymatch impedance of a power amplifier and antenna to achieve maximumpower transfer can be a challenge.

Ideally, inductors and capacitors used in a matching circuit would haveno resistance and the matching circuit would therefore dissipate littleor no power. But, in reality, a matching circuit can dissipate severalpercent of the power being transferred to the load or antenna. Thus, tominimize dissipation of power it is generally desirable to keepinductors'and capacitors'values as small as possible.

A particular problem with conventional matching networks used inwireless communications systems is that the transceivers or transmittersmust often operate at multiple frequencies. For example, a dual bandGSM/DCS radiotelephone handset uses the Global System for MobileCommunications (GSM) standard around 900 MHz, and the DigitalCommunications System (DCS) standard, which is similar to GSM exceptthat it operates around 1800 MHz.

Accordingly, there is a need for a matching circuit and method that canprovide equal or substantially equal impedance between an amplifiercircuit and a transmission circuit, thereby increasing power efficiencyand reducing signal distortion. There is a further need for a matchingcircuit and method with the ability to match impedance for signals atmultiple frequencies. It is further desirable that the matching circuitand method reduce the number or size of components used in the matchingcircuit, including the length of any transmission line used.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for matchingoutput impedance of an amplifier in a wireless communication system toan input impedance of a transmission circuit.

In one aspect, the invention provides method, apparatus, and circuit forimpedance matching using lumped and distributed inductance. In anotheraspect, this is applied to an amplifier using lumped and distributedinductance. Using a transmission line with a fixed inductor allows fornon-standard L values to be realized; furthermore, the fixed inductorallows for a shorter transmission line than were a transmission linealone to be used.

In another aspect, the invention provides an impedance matching circuithaving an input adapted for receiving a signal from an amplifier circuitand an output adapted for coupling the signal to a transmission circuit.The impedance matching circuit includes a capacitive element (C.sub.1)electrically coupled to and in parallel without the output of theimpedance matching circuit, and a series combination of an inductor(L.sub.1) and a transmission line (T.sub.1) electrically coupled betweenand in series with the input and the output. Capacitance andtransmission line lengths are selected such that in combination with theinductor a predetermined amount of total inductance L is provided.Generally, L and C are selected to provide an impedance equal to orsubstantially equal to an input impedance of the transmission circuit ata first frequency (f.sub.1) of a signal transmitted from or received bythe amplifier circuit. Advantageously, the impedance is equal, but inpractice a small deviation from equality may be acceptable as even thissubstantial equality provides better matching than conventionalapproaches.

In one embodiment, the inductor has a first end electrically connectedto an output of the amplifier circuit and a second end electricallyconnected to the transmission line. The transmission line has a firstend electrically connected to the second end of the inductor and secondend electrically connected to the output of the impedance matchingcircuit. The capacitive element is electrically connected to thetransmission line such that the length of transmission line between thesecond end of the inductor and the capacitive element provides, incombination with the inductor, the desired inductance.

In one embodiment, the transmission line is advantageously implementedusing a conductor over a ground plane on a printed circuit board (PCB),and the capacitive element is a chip capacitor mounted on the printedcircuit board. The transmission line of this type may even moreadvantageously be implemented as a co-planar grounded waveguide, and oneend or lead of the chip capacitor is soldered (or otherwise electricallyconnected) to the ground plane on the printed circuit board.Alternatively, the capacitive element is a segment of shorted waveguidebranching off from the transmission line at a point selected to providethe appropriate length to provide the desired total inductance.

In another embodiment, the impedance matching circuit further includes asecond capacitive element (C₂) electrically connected to thetransmission line and in parallel with the first capacitive element, anda switch (S₁) through which the second capacitive element is coupled toground. When opened, the switch decouples the second capacitive elementfrom ground, thereby effectively removing the second capacitive elementfrom the circuit. The second capacitive element is electricallyconnected to the transmission line such that the length of thetransmission line between the second end of the inductor and the secondcapacitive element is selected to, in combination with the inductor,provide a second predetermined amount of inductance, L′, such that L′and C are selected to provide an impedance equal or substantially equalto the input impedance of the transmission circuit at a second,generally higher frequency (f₂). In yet another embodiment, theimpedance matching circuit further includes a second switch (S₂) capableof electrically decoupling the first capacitive element from ground toremove it from the impedance matching circuit when the first switch isclosed coupling the second capacitive element to ground and introducingit into the circuit. Optionally, the first switch is a single poledouble throw switch capable of alternately electrically decoupling C₁and C₂ to ground to alternately remove C₁ or C₂ from the impedancematching circuit.

The present invention is particularly useful in a transceiver for use ina wireless communication system. Generally, the transceiver furtherincludes an amplifier circuit adapted to amplify signals received andtransmitted by the transceiver, and a transmission circuit, including anantenna, adapted to receive and transmit signals received andtransmitted by the transceiver. The impedance matching circuit iselectrically coupled between and in series with an output of theamplifier circuit and an input of the transmission circuit.

In another aspect a method is provided for matching an output impedanceof an amplifier circuit to an input impedance of a transmission circuit.In general, the method involves providing a series combination of aninductor (L₁) and a transmission line (T₁) coupled between and in serieswith an output of the amplifier circuit and an input of the transmissioncircuit. The transmission line is selected to have a length that, incombination with the inductor, provides a predetermined amount of totalinductance, L. A capacitive element (C₁) is electrically coupled to andin parallel with the output of the amplifier circuit, the capacitiveelement having a capacitance C, selected such that L and C provide theamplifier circuit with an output impedance substantially equal to aninput impedance of the transmission circuit at a frequency (f₁).

In one embodiment, the inductor has a first end connected to an outputof the amplifier circuit and a second end connected to the transmissionline, and the transmission line has a first end connected to the secondend of the inductor and second end connected to the output of theimpedance matching circuit. The step of electrically coupling thecapacitive element to and in parallel with the output of the amplifiercircuit involves the step of electrically connected the capacitiveelement to the transmission line such that the length of thetransmission line between the second end of the inductor and thecapacitive element is selected to provide the desired amount ofinductance.

In one embodiment, the transmission line includes an inductor over aground plane on a printed circuit board (PCB), and the step ofelectrically coupling the capacitive element to the output of theamplifier circuit involves mounting a chip capacitor on the PCB suchthat a first end or lead o the capacitor is electrically connected tothe conductor of the transmission line and a second end of the capacitoris electrically connected to the ground plane. Preferably, thetransmission line is a co-planar grounded waveguide, and one end of thechip capacitor is soldered to the conductor of the waveguide and theother end is soldered (or otherwise electrically connected) to theground plane on the printed circuit board.

In another embodiment, the method includes the further steps ofelectrically connecting a first end or lead of a second capacitiveelement (C₂) to the transmission line and electrically coupling theother end of the second capacitive element to the ground plane through aswitch (S₁). The second capacitive element is connected in parallel withthe first capacitive element and the output of the amplifier circuitsuch that the length of the transmission line between the end of theinductor and the second capacitive element provides a secondpredetermined amount of inductance, L′, such that L′ and C provide animpedance equal to the input impedance of the transmission circuit at asecond, typically higher frequency (f₂).

In yet another embodiment, the impedance matching circuit furtherincludes a second switch (S2) capable of electrically decoupling thefirst capacitive element, C₁, from the ground plane, thereby removing itfrom the circuit. Alternatively, the impedance matching circuit caninclude a solitary single pole double throw switch capable ofalternately electrically decoupling C₁ and C₂ from the ground plane,thereby alternately removing C₁ or C₂ from the circuit, and the step ofelectrically coupling the second capacitive element to the ground planeinvolves electrically decoupling the first capacitive element from theground plane.

The advantages of the present invention include: (i) improved impedancematching between the amplifier circuit and the transmission circuitresulting in increased power efficiency and reduced signal distortion;(ii) elimination of, or reduction in size and inductance values for, aninductor used in the matching circuit, resulting in decreased costs;and, (iii) ability to match impedance for signals at multiplefrequencies by switching a number of capacitive elements coupled todifferent points on the transmission line in and out of the circuit.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be further understood from the followingdescription in conjunction with the appended drawing. In the drawing:

FIG. 1 is an exemplary block diagram showing one example of a type of awireless communication system with which an apparatus and methodaccording to an embodiment of the present invention may be used;

FIG. 2 is a block diagram of a transceiver having an impedance matchingcircuit according to an embodiment of the present invention

FIG. 3 is a schematic diagram of an amplifier and an impedance matchingcircuit according to an embodiment of the present invention;

FIG. 4 illustrates a Smith Chart for an impedance matching circuitsimilar to that shown in FIG. 3;

FIG. 5 is a schematic diagram of an amplifier and an exemplary impedancematching circuit according to another embodiment of the presentinvention;

FIG. 6 is a schematic diagram of a dual frequency amplifier and anexemplary impedance matching circuit according to yet another embodimentof the present invention;

FIG. 7 is a schematic diagram of an alternative embodiment of theamplifier and the impedance matching circuit of FIG. 6;

FIG. 8 is a flowchart of a method for matching an output impedance of anamplifier to an input impedance of an antenna according to theembodiment of the present invention; and

FIG. 9 illustrates three equivalent impedance matching circuits in aspecific example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to an impedance matching circuit andmethod for matching an output impedance of an amplifier circuit.

FIG. 1 shows an exemplary embodiment block diagram of a wirelesscommunication system 100, here a mobile telecommunication system, forwhich an apparatus and method according to an embodiment of theinvention are particularly useful. Details of mobile telecommunicationsystems are widely known and will not be described further herein.

Referring to FIG. 1, the wireless communication system 100 generallyincludes a number of first wireless communication devices, for examplemobile handsets 105, and one or more second wireless communicationdevices, or base stations 110, distributed over a geographic area toform cells 115. The base stations 110 are coupled through base stationcontrollers 120 to a switching center 125 that is connected to a publicswitched telephone network (PSTN 130) and routes telephone calls to thebase stations covering a cell 115 occupied by the called or callinghandset 105. To enable the system 100 to locate a particular handset105, each transceiver, that is each base station 110 and handset, in thesystem has its own unique identifying number. For example, each basestation 110 has an area identity number that it transmits regularly aspart of the system's control information. Upon switching on, a handset105 will lock onto the signal of the nearest base station 110 andidentify itself to the system 100 by transmitting a registration numberto the base station. As it moves from cell to cell, the handset 105selects new base stations 110 to lock onto the handset 105 checks thearea identity number transmitted by the base station 110, and when itdetects a change indicating that it has moved to a new cell 115, it willautomatically inform the system 100 of its new location by means of asignaling interchange with the base station. In this way, the system cankeep a record (registration) of the current cell 115 in which eachhandset 105 is located, and therefore will only need to call the handsetwithin that cell.

A transceiver or transmitter 135 for use in the mobile handsets 105,base stations 110, base station controllers 120 or switching center 125of the above wireless communication system 100 will now be describedwith reference to FIG. 2. As shown in FIG. 2, transmitter 135 has animpedance matching network or matching circuit 140 according to anembodiment of the present invention which can be advantageously used toprovide an output impedance to an amplifier circuit 145 that is equal toor substantially equal to an input impedance of a transmission circuit150. Details of transmitters 135 are widely known and will not bedescribed herein.

Transmitters 135 may further include a modulator 155 to modulate a lowpower carrier wave signal, and a frequency multiplier 160 to raise thefrequency of the modulated signal the multiplier 160 raises thefrequency of the modulated signal, enabling the modulator 155 andsubsequent amplifier circuits 145 or stages to operate at differentfrequencies. The frequency-multiplied signal is then passed through aseries of amplifier circuits, only one of which is shown, to raisesignal and amplitude and to filter the signal by attenuating orsuppressing undesired frequencies. Transmitter 135 filter includes asingle amplifier circuit 145 having an amplifier and a filter (notshown), coupled to the transmission circuit 150 by an embodiment of animpedance matching circuit 140 according to the present invention. Thetransmission circuit 150 includes a transmission line 170 and an antenna175 to broadcast the signal.

An embodiment of the impedance matching circuit 140 will now bedescribed with reference to FIG. 3 which depicts an amplifier 180 foundwithin amplifier circuit 145 (see FIG. 2), and the impedance matchingcircuit 140 according to an embodiment of the present invention. Theamplifier 180 typically includes an amplifying or active element 185,such as a bipolar or field effect transistor, having an input 190 and anoutput 195 coupled through an inductor 200 to a voltage source 205,shown here as V_(DD). While shown here as a single active element 185,it will be appreciated that the amplifier 180 can include any number ofactive elements, formed either as discrete elements or as an integratedcircuit (IC), and cascaded or otherwise combined to further increasegain of the stage.

Generally, the impedance matching circuit 140 includes an input node 210coupled to the output 195 of the active element 185 and an output node215 coupled to the transmission circuit 150 (not shown in this figure).Circuit 180 further includes a capacitive element (C.sub.1) 220 having apredetermined amount of capacitance, C, electrically coupled in parallelwith the output node 215. The matching circuit 140 further includes aseries combination of an inductor (L.sub.1) 225 and a transmission line(T.sub.1) 230 electrically coupled between the input node 210 and theoutput node 215. The transmission line has a length selected to providea predetermined amount of inductance, L in combination with the inductor225. While shown as separate and distinct from electrical pathwaysconnecting the matching circuit 140 to the output 195 of the activeelement 185, and the inductor 225 to the capacitive element 220, it willbe appreciated that the transmission line 230 includes the full lengthof the transmission line extending from output 295 to the capacitiveelement 220. That is, the transmission line 230 includes line lengthfrom the output 195 to the inductor 225, and from the inductor to thecapacitive element 220. This length of the transmission line is selectedto provide, in combination with the inductor, a predetermined amount ofinductance, L, at a frequency (f.sub.1) of the signal being transmitted.Inductance L and capacitance C are selected, in view of the frequency ofinterest to provide an output impedance for the amplifier circuit 145that is substantially equal to an input impedance of the transmissioncircuit 150 so that the desired level of matching is achieved.

As noted above, the amplifier circuit 145 can include a filter prior tothe matching circuit 140 to remove undesired frequencies in the outputin a preferred embodiment, the values of the inductance L andcapacitance C of the matching circuit 140 are also selected in view ofthe signal frequency to filter undesired frequencies (if present). Thefilter can be a high pass filter (HPF) passing only those frequenciesabove a predetermined minimum frequency, a low pass filter (OPF) passingfrequencies below a predetermined maximum frequency, or a band passfilter (BPF) designed to pass only those frequencies in a predeterminedrange of frequencies. The filter may also or alternatively be designedto pass or block selected frequency ranges. For example, properselection of the values of the inductance L and capacitance C of thematching circuit 140 shown in FIG. 3 can provide a single pole LPFattenuating or suppressing all frequencies above a predetermined maximumfrequency.

The present invention advantageously permits tailoring the inductance Lof the matching circuit 140 to precisely match impedance of theamplifier circuit 145 to the transmission circuit 150, withoutlimitation as to available standard inductance values. Thus, maximumpower transfer between the amplifier circuit and the transmissioncircuit can be implemented using standard value components. A furtheradvantage is the size reduction in value of the inductor 225, whichfurther improves the efficiency of the matching circuit 140 by reducingthe amount of power dissipated in the matching circuit.

If desired, inductor 225 can be eliminated entirely by proper selectionof the length of the transmission line 230, thereby further improvingthe efficiency of the matching circuit 140, and reducing manufacturingcost.

The ability of the present invention to precisely match the outputimpedance of the amplifier circuit 145 to the transmission circuit 150will now be described with reference to the Smith Chart shown in FIG. 4.A Smith Chart is a polar plot of circles representing constantresistance, such as that of the transmission line 230, and arcsrepresenting constant reactance, such as that of the inductor 225 andcapacitive element 220. FIG. 4 shows graphed values for inductance andcapacitance for the inductor 225, transmission line 230, and thecapacitive element 220 of an impedance matching circuit 140 similar tothat shown in FIG. 3. Line 232 represents the normalized capacitance ofcapacitive element 220, line 234 represents the normalized inductance ofinductor 225, and line 236 represents the electrical length of thetransmission line 230. It will be appreciated that by varying the valuesof the capacitive element 220, the inductor 225, and/or the length ofthe transmission line 230, matching circuit 140 enables preciseimpedance matching. For example, to match the typically 3 ohm outputimpedance of a power amplifier used in a handset 105 of a wirelesscommunication system 100 to an antenna, one selects the ideal 50 ohmimpedance components values and length of the transmission line 230length such that the plot of the matching circuit terminates in a rangeof from about 0.03 to about 0.09 along the horizontal axis of the SmithChart, indicated as region 232 in FIG. 4.

FIG. 9 illustrates three equivalent 3 ohm to 50 ohm matching circuitsfor a frequency of 1 GHz. The first circuit uses lumped elements only,including a series inductor having a value of about 1.9 nH and a shuntcapacitor having a value of about 12.4 pF. In the second circuit, thelumped inductor has been replaced by a transmission line having a lengthof 11.3 mm. In the third circuit, the lumped inductor of the firstcircuit has been replaced by the combination of a shorter transmissionline (4.6 mm) and a smaller-valued inductor (1.0 nH). The shortertransmission line allows for a more compact realization, and the smallerinductor minimizes losses.

Alternative embodiments for the matching circuit 140 will now bedescribed with reference to FIG. 5, FIG. 6, and FIG. 7.

In the embodiment of FIG. 5 impedance matching circuit 140 has atransmission line 230 that includes a trace 145 on a printed circuitboard (PCB). Length of the transmission line is determined by selectingthe point at which the capacitive element 220 is electrically connectedto the trace on the PCB. Preferably, the transmission line 230 includesa conductor trace 145 over a ground plane 149 on the PCB 147 where thecapacitive element 220 is mounted on the PCB and has one end or terminalelectrically connected to the conductor and another terminalelectrically connected to the ground plane. More preferably,transmission line 230 is a co-planar grounded waveguide on a PCB, andthe capacitive element 220 is a chip capacitor soldered thereto.

FIG. 6 is a schematic diagram of an impedance matching circuit accordingto yet another embodiment of the present invention suitable for use witha dual frequency amplifier circuit. In this embodiment, the matchingcircuit 140 includes a second capacitive element 240 switchingly coupledparallel with the first capacitive element 220 by a switch 245. Thesecond capacitive element 240 has a capacitance C′ and is coupled to thetransmission line 230 at a location different than where first capacitor270 is coupled to the line. The length of the transmission line 230between the inductor 225 and the second capacitive element 240 isselected in combination with the inductor to provide a secondpredetermined inductance, L′, such that L′ and C′ match the impedance ofthe amplifier circuit 145 and the transmission circuit 150 at a secondfrequency (f₂). This embodiment is particularly useful in dual bandGSM/DCS radiotelephone handsets using the Global System for MobileCommunications (GSM) standard around 900 MHz, and the DigitalCommunications System (DCS) standard around 1800 MHz. Thus, the circuitwill provide impedance matching at two frequencies, depending uponwhether switch 245 is open or closed.

Optionally, the matching circuit 140 further includes a second switch250 that can electrically decouple the first capacitive element 220 fromthe impedance matching circuit.

To take a specific example of dual-band operation at 900 and 1800 MHz,an amplifying circuit having an output impedance of 3 ohms and atransmission circuit having an input impedance of 50 ohms, the inductor225 may have a value of 0.82 nH, and the capacitors 220 and 240 may havevalues of 2.6 pF and 10 pF, respectively. The switch 250 is assumed toalways be closed. The length of the transmission line may be 1.9 mm fromthe beginning to the capacitor 220, and 7.4 mm between the capacitors220 and 240. The switch 245 is assumed to be closed for 900 MHz and openfor 1800 MHz.

Although the switches 245, 250 are shown connected between therespective capacitive elements 220, 240 and ground, it will beappreciated that one or both of the switches could alternatively beconnected between the capacitive elements and the transmission line 230to remove the associated capacitor from the matching circuit 140. In yetanother alternative embodiment, a solitary single pole double throwswitch 255 can be used, as shown in FIG. 7, to alternately electricallycouple and decouple the first and second capacitive elements 220, 240from the impedance matching circuit 140, to impedance match at afrequency determined by the switch connections.

A method or process for operating the impedance matching circuit 140will now be described with reference to FIG. 8. FIG. 8 shows adiagrammatic flowchart of a method for matching the output impedance ofthe amplifier circuit 145 to the input impedance of the transmissioncircuit 150 according to an embodiment of the present invention. Themethod generally involves selecting a length of transmission line 230 tohave a predetermined amount of inductance, at step 265. Step 270involves coupling the transmission line and an inductor 225 in seriesbetween the input 210 and the output 215 of the matching circuit 140. Atstep 275, a capacitive element 220 having a capacitance C is coupled tothe transmission line, in parallel with the output of the matchingcircuit. At step 275, the length of the transmission line between theinductor and the capacitive element is selected to provide a totalinductance L, in combination with the inductor. The capacitive element220, inductor 225 and the length of the transmission line 230 areselected such that the values of L and C provide an output impedanceequal to an input impedance of the transmission circuit 150 for a signalat a first frequency (f₁). Preferably, where the transmission line 230is a conductor trace on a printed circuit board (PCB), step 275 ofelectrically coupling the capacitive element 220 to the transmissionline includes mounting a capacitor on the PCB such that a first lead ofthe capacitor is electrically connected to the conductor and a secondend of the capacitor is electrically connected to ground. Morepreferably, the transmission line is a co-planar grounded waveguideincluding a conductor over a ground plane. In such configuration, step275 of coupling the capacitive element to the output of the amplifiercircuit involves soldering one end of a chip capacitor to the conductorand electrically connecting the other end to the ground plane.

Optionally, the method can include the further steps of coupling asecond capacitive element 240 to the transmission line 230 in parallelwith the first capacitive element 220. Coupling of the transmission linebetween the inductor 225 and the second capacitive element provides asecond total inductance L′ selected to match the impedance of theamplifier circuit 145 and the transmission circuit 150 at a secondfrequency (f₂), as shown in step 280. Connecting the first and secondcapacitive elements 220, 240 to a single pole double throw switch 255that can electrically isolate one of the capacitive elements from thematching circuit 140 is shown at step 285. At method step 290, theswitch 255 is positioned to remove the first or the second capacitiveelements 220, 240 from the matching circuit 140, to change the frequencyat which matching occurs.

It will be appreciated by those of ordinary skill in the art that theinvention can be embodied in other specific forms without departing fromthe spirit or essential character thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restrictive. The scope of the invention is indicated by theappended claims rather than the foregoing description, and all changeswhich come within the meaning and range of equivalents thereof areintended to be embraced therein.

1. A method for creating an impedance matching circuit for matching impedances of first and second circuits, the first circuit being a transmission circuit having an input and the second circuit being an amplifier circuit having an output, the method comprising: determining an input impedance of the transmission circuit at a particular frequency; determining a capacitance C to be placed in parallel to the transmission circuit; and determining an overall inductance L of a circuit coupling the first circuit and the second circuit, said determining an overall inductance L including: determining an inherent inductance of a first line coupling the first circuit to the second circuit; determining an inductance of a second line coupling the capacitance to the first line; and determining an inductance to be placed between the first circuit and the second circuit.
 2. A method for creating an impedance matching circuit for matching impedances of first and second circuits at a first frequency, the first circuit being a transmission circuit having an input, the method comprising: selecting a length of transmission line to exhibit a predetermined amount of inductance L1; selecting an inductor to be placed between the first circuit and the second circuit and having a predetermined inductance L2; selecting a capacitor to be placed in parallel to the input of the first circuit and having a predetermined capacitance C2; and determining the impedance of the inductance L1, the inductance L2, and the capacitance C2 at the first frequency, wherein the impedance is substantially equal to an input impedance of the transmission circuit at the first frequency. 